Can anyone give me an idea why my compressor is only engaging in the defrost mode? The car in question is a 190e. The AC was not working for some time and I traced the problem to a defective coolant sensor. The push button unit is fine (same symptoms when checked with a new unit). Any ideas?

TIA

Tinker

Control theory changes through the years with the 190. I presume since the temp sensor was involved that your car is at least an 88 or newer. (the later system changest the cycling rate due to engine temp and at a point will discontinue the compressor totally).

All 190s (after 1984) use a compressor control relay that uses a number of signals to allow the compressor to work. Since the system works with defrost the engine signals to the compressor controller must be OK. You have a problem with the decission to cool. In defrost this is not a decission. The defrost position puts the compressor on no matter the temp selected.

The most common reason for the inside decission to be wrong is the feedback signal that the pushbutton controller gets from the mode door reostat. Untill the door gets fully closed (the system is still in heat until closed) the voltage from the potentiometer is too low. This voltage is adjustable on newer cars and has been the source of a couple bulletins. The vacuum to the door actuator or the integrety of the actuator can cause the door not to fully close and thusly not achieve the necessary output voltage. Does good strong airflow come from the center vents when in A/C mode? Is air coming from the defrost when in A/C mode? If so the doors aren't working fully due to leaks in the vacuum system.

Steve, thank you for the response. Yes the center vents give good air flow when in AC mode with nothing from the defrost vents. What voltage should I be seeing from the potentiometer when it is correctly adjusted? BTW, the car in question is a 87 190e 16v.

Tinker

Steve, I should have also asked how due you adjust potentiometer? Is there a nut or somethong on a rod like a linkage?

TIA

Tinker

There is a screw for adjustment. Access is through the center vents (they must be removed). The POT is on the top of the evap case. You will need better instructions than that to accomplish anything.

Do you have the chassis CD? They values change for the voltage. As I recall there are three different values depending upon the year. Testing needs to be done to evaluate this condition, unless you cheat.

We have made a resistor bridge that makes the right value for the output leg. (The potentiometer has power on one wire, ground on one wire and resulting output voltage on the third - to make the bridge two resistors are placed end to end with the output coming off the junction in between and the power and ground on the proper ends). In Florida its not a bad fix to just leave the bridge in. Those in colder climates will have to tell me why MB cared whether the mode door was all the way shut before allowing the A/C compressor to come on. I have placed this bridge in a number of old 190s for a cheap fix and can't see the difference in performance. Since the A/C has an on/off button it seems senseless to me.

If you want the resistances, I have them writen down at work.

once on my 92 400E, i had the same problem. I realized that the 2 plugs that go into the rear of the push-button unit were not in all the way, and for some reason the car was stuck in Defrost and it wouldnt turn off. pushing the plugs in better cured the problem, and i was happy! no $$$ spent!

Peter

Just got this news today
 

All, I was getting my AC serviced this morning thinking that I had lost enough R-12 since my last recharge 3 years ago so that the compressor would not engage unless the defrost setting was used. With the full charge and push button in AC the center vents give good air flow when in AC mode with nothing from the defrost vents but the compressor still does not engage. When we switch back to defrost, cold as a meat locker (rare as it may seem from a 87 190D). As I have come to understand this the defrost settings bypass the sensors and control of the compressor is always on usually a good supporting clue of needing more R-12 with a supporting visual of bubbles seen in the top of the reciever/dryer. The good news was I was only 12 oz short of a full charge, bad news is I am trying to determine, vacum block problem, potentiometer, or push button unit. Since I now live in the central florida I like the sounds of the resistor bridge that makes the right value for the output leg thus forcing the mode door to be in the correct position. Most of this system has been replaced with the last AC service with the exception of the vacum block, potentiometer, and push button unit. Where do we start?

Hello All,
Hoping you all can help me out here.
I have the same issue as the first poster in this thread. I just finished an overhaul of the non-working AC system, new seals, gas, etc. etc.

My issue is getting the compressor to run with anything but the defrost button. Now from this thread I have figured out about the blend door rheostat and how it has to be closed completely for the compressor to kick on. I have been able to put a voltmeter on the leads and see that I have 1.1V or so no matter what I do with the CCU. (BTW that CCU just came back from ProgRama in FL where it got new caps and soldering fixes).

I have figured the rheostat is good, when I pull vacuum on the 4/3 hose going to the blend pod, the voltage goes up to around 4.7V. I'm not sure if that is enough to kick on the compressor but nothing happens.

So I'm not sure if I have one issue or multiple. I think my vacuum block is not pulling the right vacuum on 4/3, but I'm not sure if that's bad CCU order or a bad switchover block. and when I get the voltage up manually I cant figure out why the compressor still wont fire.

I feel like I am missing something in the order of operations or misunderstanding what I'm reading but any help here would be great.

Thanks all.

Make me another user with this same issue: 1984 190D where the AC works well when in defrost (clutch engagement, good pressures, good and cold air flow from center vents), but compressor clutch will not engage under any other conditions. This is a recent problem; in May AC worked quite well and reliably.

I've read the above posts and, with my lack of electrical knowledge, don't really understand effective troubleshooting or repair. I'm plenty handy, but poor at electrical.

This car is a beater and I don't care if it's fixed "right", I just need AC. Can anyone out there offer a simple solution to bypass what sounds like a complicated system with lots of possible failure points?

I'm hot and I'm stupid; help :-)